petrology of the late cretaceous peralkaline rhyolites (pantellerite and comendite)

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Journal of Geosciences, 57 (2012), 127–141 DOI: 10.3190/jgeosci.118

Original paper

Petrology of the Late Cretaceous peralkaline rhyolites (pantellerite and comendite) from Lake Chad, Central Africa

Gbambie Isaac Bertrand MBOwOu1*, Claudial LaGMet2, Sébastien NOMaDe3, Ismaïla NGOuNOuNO4, Bernard DÉrueLLe5, Daniel OHNeNStetter6

1ÉcoledeGéologieetd’ExploitationMinière(EGEM),UniversitédeNgaoundéré,BP:454Ngaoundéré,Cameroun; [email protected] InstitutUniversitairePolytechniquedeMongo,BP:4377N’Djaména,Tchad3 LaboratoiredesSciencesduClimatetdel’Environnement,IPSL,CEA-CNRS-UVSQ,AvenuedelaTerrasse, 91198Gif-sur-YvetteCedex,France4 FacultédesSciences,UniversitédeNgaoundéré,BP:454Ngaoundéré,Cameroun5 UniversitéPierre-et-Marie-CurieetIUFMAcadémiedeVersailles,4,placeJussieu,75252Pariscedex05,France6 Centrederecherchespétrographiquesetgéochimiques,UFRA9046,15,rueNotre-Dame-des-Pauvres,B.P.20, 54501Vandoeuvre-les-Nancycedex,France*Correspondingauthor

Late Cretaceous peralkaline rhyolites (69.4 ± 0.4 Ma – 2σ analytical, 1.9 Ma full external uncertainty, 40Ar/39Ar sanidine single crystal laser dating) from Lake Chad are the oldest lavas of the “Cameroon Hot Line”. These lavas (pantellerite and comendite), consisting of quartz ± alkali feldspar ± fluoro-arfvedsonite ± augite–hedenbergite ± aegirine ± fayalite (Fa97–99) and ilmenite, are characterized by a progressive increase in the total REE contents and the magnitude of the negative Eu anomaly owing to the alkali feldspar-dominated fractionation. Two groups of peralkaline rhyolites have been distinguished according to mineralogical and geochemical data. Both, group-1 (LaN/YbN: 8.4–9.0; Zr/Nb: ~ 8) and group-2 (LaN/YbN: 10.7–12.6; Zr/Nb: ~ 6) are genetically related. The peralkaline rhyolites from Lake Chad could not be solely the product of fractional crystallization from a basaltic parental magma. The influx of F-rich fluids could have greatly modified the composition of rhyolitic magmas as attested by the presence of fluoro-arfvedsonite.

Keywords:petrology,geochemistry,Cretaceous,peralkalinerhyolites,LakeChad,CameroonLineReceived:18March2012;accepted:5June2012;handlingeditor:V.KachlíkTheonlineversionofthisarticle(doi:10.3190/jgeosci.118)containssupplementaryelectronicmaterial.

1. Introduction

Peralkaline rocks are commonly emplaced in both tec-tonomagmatic oceanic intraplate and continental domains such as in Ethiopia (Peccerillo et al. 2003, 2007; Ronga et al. 2010), Kenya (Weaver et al. 1972; Baker and Hen-age 1977; Scaillet and Macdonald 2001, 2003; Heumann and Davies 2002; Marshall et al. 2009; Macdonald and Bagiński 2009), SW Sardinia (Morra et al. 1994) and Pantelleria Island, Italy (Lowenstern and Mahood 1991; Civetta et al. 1998; Avanzinelli et al. 2004). An excellent example of such a volcanic association is provided by the rhyolites occurring along the “Cameroon Hot Line”, which were studied in its continental sector (Mbépit Mas-sif, Kapsiki Plateau, Garoua Trough, Benue Valley, Lake Chad, Bambouto; Ngounouno et al. 1997, 2000, 2003; Marzoli et al. 1999; Vicat et al. 2002; Wandji et al. 2008).

In this paper, new mineralogical, geochronological and geochemical data for peralkaline rhyolites from Lake Chad will be presented and compared to other rhyolite

occurrences from the “Cameroon Hot Line”. These data are used to constrain the nature, age and genesis of evolved lavas of the Lake Chad area.

2. Geological setting

The Lake Chad Basin is composed of both Late Creta-ceous volcanic and Cenozoic to Quaternary sedimentary formations (Vicat et al. 2002; Ganwa et al. 2009), under-lain by a Precambrian basement consisting of gneiss and migmatites (Schroeter and Gear 1973). According to the exploratory drilling at Logone-Birni and Bol (Lake Chad Basin), this basement is located at about 600 m depth (Barbeau 1956). The volcanic eruptions were probably triggered after the reactivation of Precambrian basement faults (Moreau et al. 1987) that could have been linked to the occurrence of hot lines in the asthenospheric mantle (Bonatti et al. 1976). Lake Chad Basin was an active tec-tonic pull-apart basin (a graben, according to Vicat et al.

Gbambie Isaac Bertrand Mbowou, Claudial Lagmet, Sébastien Nomade, Ismaïla Ngounouno, Bernard Déruelle, Daniel Ohnenstetter

128

HadjerBigliHadjer

Hidjer

Karal

toN

’dja

me

n

a

to

Camero

on

N

0 4 km

Chari RiverGoubagoMakalif

Guirbé

14˚50'

14˚40’

12˚50’

Lake Chad

el Hamisel Hamis

Volcanoes Late Cretaceous

Continentalsediments

Cenozoic-to-Quaternary

BenueValley

200 km

Mt. OkuMt. Oku

Mt. Manengouba

Bioko

Pagalu

Gulf

10°°

12°° 16°°

0°°

4°

6°°

Garoua

Mts Bambouto

Mbam

Golda Zuelva

Waza

Koupe

Mboutou

Mt. Etinde

PlateauKapsiki

N

Yaounde

Sanaga

Shear Zone

Chad

Poli

Plateau

T

10°

of

Guinea

6°°

2°°

14°

Lake

Cam

ero

on

Lin

e

Congo Craton

TchabalMbabo

TchabalNganhaTchabal

PrincipePrincipe

H. Guenfala

AdamawaAdamawa African

Shear ZoneCentral

SăSăo Tomé

Rumpi H.

Chari

LogoneAscension

St. Helena

Atlantic Ocean

Africa

Cap desPalmes

ZermanaZermana

NgaoundéréNgaoundéré

Mbépit

to Tourba

AlmégranSalga

AdrikAkoulkoull

Abado

Roads

studied area

Doual

Lake Chad

Garoua

NganhaNganha

Rumpi Mt.

Mt. CamerounMt. Cameroun

DoualaDouala

Rumpi Mt.Rumpi Mt.

HadjerHadjer

C a m e r o o nC a m e r o o n

GarouaGaroua

8°

Hot

12°

Fig. 1 Location of the studied area, tectono-magmatic settings of the Cameroon Hot Line (Déruelle et al. 2007) and sampling sites of peralkaline rhyolites from Lake Chad.

Petrology of the peralkaline rhyolites from Lake Chad, Central africa

129

2002), located on the northern Congo Craton (Fig. 1). The tectonic plates that affected Africa and South America during the Aptian and Albian times, leading to the open-ing of the Central Atlantic Ocean (Benkhelil 1986), con-tributed to the formation in the Lake Chad Basin (Louis 1970) of Agadem and Bornu troughs (Vicat et al. 2002) and the associated volcanism.

Peralkaline rhyolites from SE of Lake Chad (Fig. 1) were sampled on rocky slopes and/or in working quar-ries at the flanks of Hadjer el Hamis, Hadjer Bigli and Hadjer Hidjer volcanoes, at the northeastern end of the Cameroon Line (Déruelle et al. 1991). The Cameroon Line, recently named “Cameroon Hot Line”, is an ac-tive intraplate tectonomagmatic zone, with associated alkaline volcanism, developed on both oceanic and con-tinental lithosphere. Oriented N30°E, it crosses the Gulf of Guinea and Cameroon for more than 2000 km, from Pagalu Island to Lake Chad (Déruelle et al. 2007).

Hadjer el Hamis volcanoes (418 m a.s.l.) are five necks composed of dark green rhyolites, showing verti-cal columnar jointing. The rhyolitic fragments split from the flanks are scattered around the foothills. The Hadjer Bigli plug (308 m a.s.l.) is made up of strongly weathered reddish brown rhyolite, which is cut by greenish dark rhyolitic dykes. This volcano of c. 15–20 m height is slightly flattened on its top. Hadjer Hidjer (283 m a.s.l.) is a flattened and slightly elongated (N30°E) plug–dome surrounded by continental sedimentary rocks, which is made up of greenish dark rhyolite belonging to the inner core of the small lava plug. Its surface is covered by lava blocks, broken along columnar and tabular joints, which were likely produced by the internal motion of the highly viscous magma.

3. Analytical methods

Mineral phases (olivine, clinopyroxene, amphibole, alkali feldspars, Fe–Ti oxides, quartz) have been analyzed by electron microprobes CAMEBAX SX50 and SX100 at Université Pierre-et-Marie Curie, Paris. The measure-ments were made according to standard analyzed data,

under the conditions expressed in kV (accelerating voltage), nA (beam current) and s (counting times at the peak). Olivine (15 kV, 40 nA, 20 s for all elements, except Si (10 s)), clinopyroxene (15 kV, 40 nA, 20 s for Si, Al, Fe, Mg, Ca, Na, Mn and 30 s for Ti and Zr), amphibole (15 kV, 10 nA, 15 s for Si, Al, Mg, Na and K, 20 s for Ca and Ti, 25 s for Fe and Mn, 30 s for Cl and F), feldspar (15 kV, 10 nA, 5 s for all elements), Fe–Tioxides (15 kV, 40 nA, 40 s for Ti, Fe, Mn, Mg; 10 s for Si, 15 s for Cr and 30 s for Al). Measurements correction was carried out using the “PAP” program (Pouchou and Pichoir 1991). Analyses are given in terms of oxides of the elements (wt. %). The recalculations of amphibole formulae were undertaken using the script of Tindle and Webb (1994).

Whole-rock chemical analyses of peralkaline rhyolites from Lake Chad were carried out at CRPG laboratory, Nancy. Major elements were analyzed by ICP-AES and trace elements by ICP-MS. The samples were previously selected in order to limit superficial contamination, then crushed. Details of other analytical processes were pre-sented elsewhere (Carignan et al. 2001).

The dating of a peralkaline rhyolite sample from Lake Chad (Hadjer el Hamis, Tab. 1) was performed by 40Ar/39Ar method at Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS-UVSQ, France. For 40Ar/39Ar dating, pristine sanidine crystals 200–250 µm in size were handpicked under a binocular and slightly leached for 5 minutes in a 5% HF acid solution. A total of 15 single crystals were finally handpicked after leaching and separately loaded in a single pit in an aluminum disk. Sample was irradiated for 90 minutes (Irr 14) in the β1 tube of the OSIRIS reactor (CEA Saclay, France). After irradiation, crystals were transferred one by one into a copper sample holder and then loaded into a differential vacuum Cleartran© window. A total of 10 crystals were individually fused at about 12 % of the full laser power. Argon isotopes were analyzed using a VG 5400 mass spectrometer equipped with a single ion counter (Balzers© SEV 217 SEN) following procedures outlined in Nomade et al. (2010). Each Ar isotope measurement consisted of 20 cycles of peak switching of the argon isotopes. Neu-

Tab. 1 Overview of geochronological data for rhyolites from the “Cameroon Hot Line”

location lavas analytical methods ages (Ma) references

Mbépit Massif rhyoliterhyolite

K–ArK–Ar

45.5 ± 1.144.0 ± 1.0

Wandji et al. (2008)Wandji et al. (2008)

Mts Bambouto rhyolite K–Ar 16.0 Marzoli et al. (1999)

Mt. Oku rhyoliteignimbritic rhyolite

K–Ar K–Ar

23.224.8 ± 0.1

Dunlop (1983)Dunlop (1983)

Benue valley rhyolite K–Ar 36.8 ± 0.9 Ngounouno et al. (2003)

Kapsiki Plateau rhyoliterhyolite

Rb–SrRb–Sr

29 ± 0.532 ± 0.5

Dunlop (1983)Dunlop (1983)

Lake Chad peralkaline rhyoliteperalkaline rhyolite

K–ArAr–Ar

68.9 ± 1.469.4 ± 0.4

Schroeter and Gear (1973)This study


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tron fluence (J) was monitored by co-irradiation of Alder Creek sanidine (ACs, Nomade et al. 2005) placed in the same pit as the leucite. The J value was determined from analyses of three single ACs crystals. Corresponding J value (see online electronic supplement, Appendix 1, for full data table) was calculated using an age of 1.193 Ma (Nomade et al. 2005) and the total decay constant of Steiger and Jäger (1977). The precision and accuracy of the mass discrimination correction was monitored by daily measurements of air argon (see full experimental description in Nomade et al. 2010).

4. Petrography

4.1. Hadjer Bigli (FB)

The main body of the Hadjer Bigli plug is composed of reddish brown rhyolites, with partly welded tuff appearance. These rhyolites display quartz and alkali feldspar phenocrysts set in an altered devitrified glassy groundmass, indicating late to post-magmatic altera-tion. Alkali feldspar is euhedral to subhedral, present as both phenocrysts and groundmass phase. Some alkali feldspar phenocrysts are altered and crossed by brown bands. Quartz occurs as euhedral phenocrysts (2.1–1.2 mm). The greenish dark rhyolite dykes from Hadjer Bigli are microlithic porphyritic, with quartz (1–2 mm), alkali feldspars (~ 1.5 mm), green amphibole (0.4–1.5 mm), clinopyroxene (~ 0.5 mm) and Fe–Ti oxide (~ 0.9 mm) phenocrysts, in a groundmass consisting of alkali feldspar, amphibole, clinopyroxene and Fe–Ti oxide mi-crolites, unevenly distributed in the glass. The quartz phenocrysts are sometimes agglomerated and Fe–Ti oxide inclusions are present in the alkali feldspar phenocrysts.

4.2. Hadjer el Hamis (De)

Rhyolitic samples from Hadjer el Hamis are porphyritic and partly brecciated. They contain phenocrysts of quartz (up to 2.5 mm), alkali feldspar (~ 3.5 mm), amphibole, clinopyroxene and Fe–Ti oxide, in a groundmass con-sisting of alkali feldspar, greenish amphibole and Fe–Ti oxides microlites enclosed by a thin glassy phase. Quartz phenocrysts are euhedral, with corrosion gulfs. Clinopy-roxene phenocrysts are overgrown and partly replaced by Fe–Ti oxides. Several small (1.5 cm) basaltic xenoliths occur in these samples. They are strongly altered and carry scarce phenocrysts of mafic minerals (e.g., oliv-ine and clinopyroxene) and clear plagioclase which are scattered in the dark-brownish glassy groundmass. The mafic phenocrysts are sometimes pseudomorphosed by fine-grained Fe–Ti oxides.

4.3. Hadjer Hidjer (DH)

The porphyritic rhyolite from Hadjer Hidjer is partly brecciated and contains xenoliths of basaltic fragments. Quartz (1.3 mm), alkali feldspar (3 mm), clinopyroxene, amphibole (1.8 mm), Fe–Ti oxide (0.5 mm) phenocrysts and xenoliths of basalt (see Ganwa et al. 2009) are en-closed in a groundmass of amphibole, clinopyroxene and Fe–Ti oxides microlites. The pleochroic amphibole phenocrysts are scattered and sometimes show olivine inclusions enclosed in a reddish brown core, frequently altered to Fe–Ti oxides.

5. Mineral chemistry data

Electron microprobe compositions of representative mineral phases of rhyolites from Lake Chad are re-ported in Tabs 2–6. Compositional variation of py-roxene, amphibole and feldspar are illustrated in Figs 2a–b to 3.

Wo

En Fs

Augite

Diopside

Pigeonite

EnstatiteEnstatite FerrosiliteFerrosillite

HedenbergiteHedenbergite

0.0

0.5

1.0

mg

#

9.0 8.5 8.0 7.5 7.0 6.5

Si (a.p.f.u.)

ferro-eckermannite

arfvedsonite

eckermannite

magnesio-arfvedsonite

nyböite

ferric-nyböite

ferronyböite

ferric-ferronyböite

(AlVI ≥ Fe3+) (AlVI ≥ Fe3+)

(AlVI < Fe3+) (AlVI < Fe3+)

(AlVI ≥ Fe3+)

(AlVI < Fe3+)(AlVI ≥ Fe3+)

(AlVI < Fe3+)

Hadjer Hidjer

Hadjer Bigli

Hadjer el Hamis

b

a

Fig. 2 Compositional variations of ferromagnesian minerals from the Lake Chad rhyolites: a – classification of clinopyroxene in the Wo–En–Fs ternary diagram (Morimoto 1989); b – classification diagram mg# vs Si (according to Leake et al. 1997) for amphibole.


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5.1. Olivine

Fayalite-rich olivine (Fa97–99) was analyzed in rhyolites from Hadjer el Hamis and Hadjer Hidjer (Tab. 2). The CaO contents are high (up to 0.76 wt. %), indicating low crystallization pressures near the surface (Simkin and Smith 1970), at low temperatures (<900 °C, estimated after Ro-eder and Emslie 1970). Fayalite phenocrysts are partly or widely replaced by fine-grained aggregates of Fe–Ti oxides.

5.2. Clinopyroxene

Clinopyroxene (Tab. 3) analyses straddle the boundary of the hedenbergite and augite fields (Fig. 2a). The TiO2 contents range from 0.35 to 1.00 wt. %. At Hadjer Hidjer occur rare phenocrysts of aegirine–augite, characterized by high TiO2 contents (7.3 wt. %) together with augite–hedenbergite. On the other hand, at Hadjer Bigli were recorded aegirine phenocrysts with 0.89 wt. % TiO2. High TiO2 contents in sodic clinopyroxene indicate low crystal-

Tab. 2 Selected compositions of olivine from peralkaline rhyolites of Lake Chad (wt. % and a.p.f.u. on the basis of 4 oxygens)

Sample DH3 DH3 DEdescription ph ph phSiO2 (wt. %) 30.41 39.24 38.65Al2O3 0.00 0.09 0.01FeO 64.95 56.36 55.39MnO 3.00 2.16 3.90MgO 0.95 0.35 0.34CaO 0.44 0.76 0.66Sum 99.75 98.96 98.95Si (a.p.f.u.) 1.017 1.217 1.206Al 0.000 0.003 0.000Fe2+ 1.817 1.462 1.446Mn 0.085 0.057 0.103Mg 0.047 0.016 0.016Ca 0.016 0.025 0.022Fo (mol. %) 2.44 1.06 1.01Fa 97.56 98.95 98.99

ph: phenocrysts; a.p.f.u.: atom per formula unit

Tab. 3 Clinopyroxene compositions (wt. % and a.p.f.u. on the basis of 6 oxygens)

Sample DH2 DH3 DH3 DH4 DH4 DH4 DE DE DE DE DE DH4 FB1description ph ph,c ph,c ph,r ph,r ph,c ph,c ph,c ph,c ph,r ph,c ph phSiO2 (wt. %) 49.20 48.37 48.98 49.61 49.67 48.52 49.34 48.43 46.93 48.21 48.71 41.56 53.1TiO2 0.43 0.39 0.42 0.37 0.24 0.40 0.63 0.32 1.00 0.35 0.38 7.34 0.89Al2O3 0.29 0.17 0.16 0.16 0.19 0.47 0.68 0.11 1.00 0.15 0.16 0.06 0.15FeO 26.48 28.38 28.46 29.11 28.66 26.2 22.64 28.94 26.04 28.67 28.99 40.69 30.56MnO 1.06 1.27 1.19 1.20 0.99 1.14 1.36 1.02 1.28 1.01 1.07 1.42 0.30MgO 1.47 0.77 0.73 1.01 1.05 3.29 5.21 1.09 2.32 0.71 1.12 0.14 0,00CaO 18.96 18.36 17.82 15.51 15.99 18.65 19.14 16.42 20.03 18.33 17.12 0.15 0.37Na2O 1.26 1.41 1.39 2.33 2.95 1.06 0.47 2.22 0.50 2.02 1.78 7.14 14.31ZrO2 0.03 0.06 0.06Sum 99.15 99.12 99.18 99.30 99.74 99.73 99.47 98.55 99.16 99.51 99.33 98.50 99.68Fe2O3 (calc.) 1.20 3.23 1.43 3.49 6.76 4.05 0.48 5.60 2.70 6.49 4.07 21.53 37.11FeO (calc.) 25.40 25.48 27.17 25.97 22.58 22.56 22.20 23.90 23.61 22.83 25.33 21.32 2.83Sum (calc.) 99.27 99.44 99.32 99.65 100.42 100.14 99.52 99.11 99.43 100.16 99.74 100.66 100.57Si (a.p.f.u.) 2.011 1.990 2.016 2.023 2.001 1.957 1.976 1.989 1.923 1.965 1.992 1.721 1.969Ti 0.013 0.012 0.013 0.011 0.007 0.012 0.019 0.009 0.031 0.011 0.012 0.229 0.025Al 0.014 0.008 0.008 0.008 0.009 0.022 0.032 0.005 0.048 0.007 0.008 0.003 0.007Fe3+ 0.037 0.099 0.044 0.107 0.205 0.123 0.015 0.173 0.083 0.199 0.125 0.671 1.035Fe2+ 0.868 0.877 0.935 0.886 0.761 0.761 0.744 0.821 0.809 0.778 0.866 0.738 0.000Mn 0.037 0.044 0.042 0.042 0.034 0.039 0.046 0.036 0.044 0.035 0.037 0.049 0.009Mg 0.089 0.047 0.045 0.061 0.063 0.198 0.311 0.067 0.142 0.043 0.068 0.009 0,000Ca 0.830 0.809 0.786 0.678 0.690 0.806 0.821 0.723 0.879 0.801 0.750 0.007 0.015Na 0.099 0.113 0.111 0.184 0.230 0.083 0.037 0.177 0.040 0.159 0.141 0.573 1.029Zr 0.001 0.001 0.001Fe3+/Fe2+ 0.043 0.113 0.047 0.121 0.269 0.162 0.020 0.211 0.103 0.256 0.144 0.909Wo (mol. %) 45.43 45.30 43.20 40.03 44.45 43.68 42.20 43.60 45.64 47.77 43.28En 4.17 2.80 1.30 2.11 3.97 11.75 17.05 4.24 8.20 2.78 4.15Fs 50.40 51.90 55.50 57.85 51.58 44.57 40.75 52.16 46.16 49.45 52.57Q 1.79 1.73 1.77 1.63 1.51 1.76 1.88 1.61 1.83 1.62 1.69 0.75 0.00J 0.20 0.23 0.22 0.37 0.46 0.17 0.07 0.35 0.08 0.32 0.28 1.15 2.06

c: core ; r: rim; DH: Hadjer Hidjer; DE: Hadjer el Hamis; FB: Hadjer Bigli


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lization temperature (<600 °C, Ferguson 1977) as showed for the aegirine from the Djinga Tadorgal lavas (Mbowou et al. 2010). The crystallization of aegirine appears to be related to the peralkaline magmatic environment and low temperature (Brousse and Rançon 1984).

5.3. amphibole

Amphibole phenocrysts (Tab. 4) are classified as arfved-sonite according to the nomenclature diagram of Leake et al. (1997; see Fig. 2b). High F contents of up to 2.9 wt. % indicate crystallization under low oxygen fugacity at a late-magmatic stage (Strong and Taylor 1984). The presence of arfvedsonite is characteristic of the most highly evolved peralkaline rocks of both under- and oversaturated suites (Mitchell and Platt 1978).

5.4. Fe–ti oxide

At Hadjer Hidjer ilmenite occurs as anhedral crystals with FeO and MnO contents reaching 45.9 wt. % and 3.2 wt. %, respectively (Tab. 5).

Tab. 4 Amphibole compositions (wt. % and a.p.f.u. on the basis of 23 oxygens)

Sample DH3 DH3 DH4 DE DE FB1 FB1 FB1 FB1description ph,c ph,r ph,r ph,c ph,c ph,c ph,c ph,c ph,cSiO2 (wt. %) 48.97 49.51 49.98 50.83 50.31 50.65 49.96 49.81 49.01TiO2 1.15 0.93 0.18 1.04 0.75 0.75 0.69 0.81 1.15Al2O3 0.77 0.37 0.28 0.35 0.40 0.34 0.31 0.33 0.74FeO 32.15 33.69 33.69 33.96 34.67 34.94 34.17 34.27 33.83MnO 1.01 1.10 1.12 0.61 0.68 0.72 0.68 0.73 0.75MgO 0.97 0.80 1.15 0.03 0.02 0.00 0.00 0.01 0.13CaO 2.95 3.01 2.88 0.74 1.02 0.70 0.60 0.95 2.52Na2O 6.75 6.90 6.71 8.04 7.84 8.34 8.09 8.13 7.35K2O 1.46 1.38 1.37 1.82 1.52 1.45 1.62 1.37 1.36F 2.45 2.54 2.85 1.78 2.06 2.33 2.47 1.93 2.16Cl 0.06 0.05 0.05 0.01 0.01 0.01 0.01 0.00 0.02F as O 1.03 1.07 1.20 0.75 0.87 0.98 1.04 0.81 0.91Cl as O 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00Sum 99.73 101.36 101.47 99.96 100.15 101.21 99.64 99.15 99.93Si (a.p.f.u) 7.965 7.951 7.994 8.159 8.080 8.085 8.119 8.084 7.977AlIV 0.035 0.049 0.006 0.000 0.000 0.000 0.000 0.000 0.023AlVI 0.112 0.022 0.046 0.066 0.076 0.064 0.059 0.063 0.119Ti 0.141 0.112 0.022 0.126 0.091 0.090 0.084 0.099 0.141Fe3+ 0.182 0.335 0.569 0.235 0.480 0.471 0.440 0.399 0.141Fe2+ 4.191 4.190 3.937 4.324 4.177 4.193 4.203 4.252 4.464Mn 0.139 0.150 0.152 0.083 0.093 0.097 0.094 0.100 0.103Mg 0.235 0.192 0.274 0.007 0.005 0.000 0.000 0.002 0.032Ca 0.514 0.518 0.494 0.127 0.176 0.120 0.104 0.165 0.439Na 2.129 2.149 2.081 2.502 2.441 2.581 2.549 2.558 2.320K 0.303 0.283 0.280 0.373 0.311 0.295 0.336 0.284 0.282F 1.260 1.290 1.442 0.904 1.046 1.176 1.269 0.991 1.112Cl 0.017 0.014 0.014 0.003 0.003 0.003 0.003 0.000 0.006OH 0.723 0.696 0.545 1.094 0.951 0.821 0.728 1.009 0.883Mg/(Mg+Fe2+) 0.053 0.044 0.065 0.002 0.001 0.000 0.000 0.001 0.007

Ab Or

An

Hadjer Hidjer

Hadjer Bigli

Hadjer el Hamis

Anorthoclase

iteAlbiteNa-Sanidine Sanidine

Fig. 3 An–Ab–Or diagram for alkali feldspar of the peralkaline rhyolites from Lake Chad.


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5.5.Alkalifeldspars

As demonstrated in the Ab–An–Or diagram (Fig. 3; see Tab. 6), there are three main groups of alkali feldspars, separated by considerable gaps: (1) nearly pure albite (Ab98–97), (2) anorthoclase–Na-sanidine (Or18–44Ab75–52An0.3–7) and (3) nearly pure sanidine (Ab98–96).

6. 40Ar–39Ar ages

The new dating on a peralka-line rhyolite (DE) from Had-jer el Hamis yielded an age of 69.4 ± 0.4 Ma (electronic supple-ment, Appendix 1), which falls within the error of the previously determined age for peralkaline rhyolites from the same dome

Tab. 5 Ilmenite compositions (wt. % and a.p.f.u. on the basis of 6 oxygens)

Sample DH3 DH3 DH3 DH3 DH4 DH4 DH4SiO2 (wt. %) 0.00 0.05 0.01 0.00 0.00 0.00 0.01TiO2 49.98 50.39 50.78 51.19 52.74 51.74 51.89Al2O3 0.01 0.09 0.04 0.02 0.00 0.00 0.00Cr2O3 0.06 0.03 0.00 0.00 0.09 0.00 0.00FeO 45.53 44.34 43.35 43.77 43.70 45.89 44.57MnO 1.62 1.44 2.27 2.40 3.20 2.22 2.40MgO 0.10 0.06 0.02 0.02 0.00 0.02 0.03CaO 0.00 0.11 0.03 0.01 0.09 0.01 0.01Sum 97.30 96.51 96.50 97.41 99.82 99.88 98.91Fe2O3 (calc.) 2.66 0.74 0.04 0.23 1.83 0.42FeO (calc.) 43.14 43.68 43.31 43.56 44.08 44.24 44.19Sum (calc.) 97.57 96.58 96.50 97.43 99.78 100.06 98.95Si (a.p.f.u) 0.000 0.003 0.001 0.000 0.000 0.000 0.001Ti 1.947 1.980 1.998 1.995 2.006 1.965 1.991Al 0.001 0.006 0.003 0.001 0.000 0.000 0.000Cr 0.003 0.001 0.000 0.000 0.004 0.000 0.000Fe3+ 0.104 0.029 0.002 0.009 0.070 0.016Fe2+ 1.868 1.908 1.894 1.888 1.864 1.868 1.885Mn 0.071 0.064 0.101 0.105 0.137 0.095 0.104Mg 0.008 0.005 0.002 0.002 0.000 0.002 0.002Ca 0.000 0.006 0.002 0.001 0.005 0.001 0.001Fe3+/Fe2+ 0.056 0.015 0.001 0.005 0.037 0.009

Lake Chad

Th

isst

ud

y

Golda ZuelvaKapsiki

Upper Benue (Nigeria)

Garoua Rift

Kokoumi

Poli TcheguiUpper Benue(Nigeria)

Mayo Darle

NkogamNlonakoBangou

Koupe

Mbépit

5

10

0

-3

0 10 20 30 40 50 60 70

EquatorSão Tomé

Pagalu

Principe

Bioko

Mt Etinde

Mt CameroonMts Rumpi

Mts BamboutoManengouba

BamendaOku

VolcanismPlutonism

Cameroon Hot Line

Age (Ma)

La

titu

de

(°N

)

Fig. 4 New age for peralkaline rhyolites from Lake Chad in context of other ages for igneous rocks from the “Cameroon Hot Line” (data from Déruelle et al. 2007 and references therein). For original data, see electronic supplement, Appendix 2.


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(68.9 ± 1.4 Ma: Schroeter and Gear 1973) (Fig. 4); This suggests that all components of the dome are contempora-neous. The likely rapid cooling of the rocks together with the freshness of the sample lead us to regard the ~69 Ma age as reflecting the Late Cretaceous emplacement of the volcanic rocks in the Lake Chad area.

7. Geochemistry

The samples analyzed in the present study fall within the rhyolite field in TAS diagram (Fig. 5a). They have peralkaline index (P.I. = molar [Na2O + K2O]/Al2O3) higher than unity. Using the Al2O3 vsFeOt plot (Fig. 5b; Macdonald 1974), peralkaline silicic lava from Hadjer el Hamis is classified as comendite, while those from Hadjer Bigli and Hadjer Hidjer straddle the boundary to the pantellerite domain. Differentiation index (D.I. = ∑Qtz+Or+Ab+Ne+Lc, CIPW norm) ranges be-tween 89.9 and 93.4, with low mg# (100 × molar MgO/[MgO + FeO

t]) values (2–6). Major- and trace-element

compositions for the studied samples are reported in Tab. 7.

7.1. Major elements

Apart from the low FeOt contents for peralkaline rhyo-lites from Hadjer el Hamis, the compositional trend from Hadjer Hidjer to Hadjer el Hamis, then to Hadjer Bigli is continuous and characterized by an increase of SiO2 from 68.2 to 74.5 wt. %, accompanied by a decrease of TiO2, Al2O3, CaO, MgO and Na2O (see Tab. 7).

7.2. trace elements

Overall, the lavas from Lake Chad are characterized by a progressive enrichment in LREE, Rb, Zr, Nb, Y and Th, whereas Ba, Sr, Co, V decrease from Hadjer Hidjer to Hadjer el Hamis, then to Hadjer Bigli (Tab. 7). The Zr/Nb ratios of peralkaline rhyolites from Hadjer Bigli (~ 6) are different from those of Hadjer Hidjer and Hadjer el Hamis (just below 8). Primitive mantle-normalized (McDonough and Sun 1995) REE patterns for all sam-ples are subparallel (Fig. 6). The main features of silicic lavas from Lake Chad, as differentiation increases from Hadjer Hidjer to Hadjer el Hamis, then to Hadjer Bigli are: (1) increase in total concentrations of REE, (2) slight increase in LREE/HREE enrichment, with LaN/YbN ratios ranging from 8.4 to 12.6, (3) development of a progres-sively deeper negative Eu anomaly. Primitive mantle-normalized (McDonough and Sun 1995) multi-element patterns (Fig. 7) show slight positive Zr and negative Ba, P, Sr and Ti anomalies. Peralkaline rhyolites from entire “Cameroon Hot Line” exhibit similar anomalies (Ngou-nouno et al. 1997, 2000, 2003; Marzoli et al. 1999; Vicat et al. 2002; Wandji et al. 2008). Additionally, a negative K anomaly characterizes the Hadjer Bigli patterns.

The silicic lavas studied form two groups: (1) peral-kaline rhyolites from Hadjer Hidjer and Hadjer el Hamis show LaN/YbN ratios relatively low, between 8.4 and 9.0, and low Zr (647–709 ppm) and Nb (84–90) concentra-tions (Zr/Nb: ~ 8). This group (group-1) corresponds to weakly peralkaline lavas characterized by the presence of fayalite-rich olivine and in terms of Zr and Nb are less differentiated rhyolites, (2) peralkaline rhyolites

Tab. 6 Alkali feldspar compositions (wt. % and a.p.f.u. on the basis of 8 oxygens)

Sample DH3 DH3 DH4 DH4 DH4 DH4 DH4 DE DE DE FB1 FB1 FB1 FB1

description ph ph ph ph ph ph ph ph ph ph ph ph ph ph

SiO2 (wt. %) 67.52 65.84 67.48 67.23 67.42 65.65 67.11 68.30 63.88 66.26 68.83 69.13 64.65 62.58

Al2O3 18.68 20.21 19.12 18.63 18.82 17.89 18.72 18.65 18.35 18.16 19.60 19.45 18.41 14.39

FeO 0.47 0.21 0.39 0.51 1.25 0.06 0.33 1.39 0.05 1.16 0.46 0.04 0.00 7.03

CaO 0.02 1.47 0.14 0.03 0.01 0.11 0.02 0.06 0.00 0.03 0.02 0.03 0.02 0.09

Na2O 7.74 8.59 7.73 8.07 5.65 0.34 6.99 11.24 0.27 6.43 11.55 11.12 0.13 1.80

K2O 5.64 3.19 5.63 5.52 7.34 15.84 6.45 0.12 16.76 7.18 0.18 0.46 16.41 12.60

Sum 100.07 99.51 100.49 99.99 100.49 99.89 99.62 99.76 99.31 99.22 100.64 100.23 99.62 98.49

Si (a.p.f.u) 3.008 2.934 2.993 2.997 3.007 3.027 3.007 3.005 2.985 3.003 2.992 3.009 2.999 2.977

Al 0.981 1.061 1.000 0.979 0.989 0.972 0.989 0.967 1.011 0.970 1.004 0.998 1.006 0.807

Fe2+ 0.018 0.008 0.014 0.000 0.047 0.002 0.012 0.051 0.000 0.038 0.017 0.001 0.000 0.110

Ca 0.001 0.070 0.007 0.001 0.000 0.005 0.001 0.003 0.000 0.001 0.001 0.001 0.001 0.005

Na 0.670 0.744 0.667 0.700 0.490 0.030 0.609 0.962 0.025 0.567 0.976 0.941 0.012 0.167

K 0.320 0.181 0.319 0.314 0.418 0.932 0.369 0.007 0.999 0.415 0.010 0.026 0.971 0.765

An (mol. %) 0.26 7.18 1.50 0.33 4.62 0.82 1.16 1.80 0.36 1.40 1.34 0.28 0.41 5.73

Ab 67.49 74.65 66.64 68.82 51.52 3.13 61.58 97.53 2.41 56.93 97.66 97.11 1.18 16.85

Or 32.25 18.17 31.86 30.85 43.86 96.05 37.26 0.67 97.23 41.67 1.00 2.61 98.40 77.42


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from Hadjer Bigli yield normalized patterns showing a high LREE/HREE enrichment (LaN/YbN = 10.7–12.6). This group (group-2) corresponds to highly peralkaline rhyolites which are characterized by the absence of fayalite-rich olivine and by relatively high HFSE contents (High Field-Strength Elements; Zr: 1926–2445 ppm; Nb: 314–414 ppm; Zr/Nb: ~ 6).

8. Discussion

8.1. Geochronological implications

Radiometric ages for plutonic and volcanic rocks along the “Cameroon Hot Line” span an interval between 0 Ma and 69 Ma (Fig. 4). The current work indicates that

1

3

5

7

9

11

13

15

41 45 49 53 57 61 65 69 73 77

Basalt

RhyoliteTrachyte

SiO2 (wt. )%

Na

2O

+ K

2O

(w

t. %

)

FeOt (wt %).

Al 2

O3

(wt

%)

.

Pantelleritictrachyte

Comenditictrachyte

Pantellerite

Comendite

0 2 4 6 8 10 12 14

5

7

9

11

13

15

17

19

b

a

Hadjer HidjerHadjer Bigli

Hadjer el Hamis

Kapsiki Plateau

Benue Valley

Lake Chad

Pantelleria area

KenyaEthiopian Rift

this study

Mbépit assifM

Fig. 5 Total alkali–silica (TAS) diagram (Le Bas et al. 1986) (a) and FeO

t –

Al2O

3 classification diagram for peralka-

line rocks (Macdonald 1974) (b). Data from Ethiopia (Peccerillo et al. 2007; Ronga et al. 2010), Kenya (Scaillet and Macdonald 2003), Pantelleria Island, Italy (Avanzinelli et al. 2004), Mbépit Massif (Wandji et al. 2008) Kapsiki Plateau, Benue Valley (Ngounouno et al. 2000, 2003) and Lake Chad (Vicat et al. 2002).


136

Tab. 7 Whole-rock major- and trace-element analyses of peralkaline rhyolites from Lake Chad

Location Hadjer Hidjer Hadjer el Hamis Hadjer BigliSample DH2 DH3 DH4 DE FB1 FB3GPScoordinates

14°39’E12°47’N

14°39’E12°47’N

14°39’E12°47’N

14°51’E12°52’N

14°40’E12°48’N

14°40’E12°48’N

SiO2 (wt. %) 68.65 68.18 68.15 70.54 75.31 74.58TiO2 0.41 0.43 0.42 0.29 0.18 0.19Al2O3 12.21 12.47 12.42 12.06 9.72 9.51FeOt 6.04 6.33 6.13 4.83 5.80 5.40MnO 0.18 0.18 0.19 0.22 0.08 0.06MgO 0.10 0.10 0.12 0.04 0.03 0.04CaO 1.04 1.01 1.11 0.31 0.27 0.24Na2O 5.34 5.36 5.32 5.00 3.82 3.28K2O 4.35 4.37 4.30 4.71 4.37 4.47P2O5 0.06 0.06 0.06 0.10 < D.L. < D.L.L.O.I. 0.31 0.44 0.30 0.74 1.31 1.71Sum 98.69 98.91 98.50 98.82 100.88 99.48D.I. 89.90 89.20 89.03 93.38 92.38 92.01P.I. 1.11 1.09 1.08 1.10 1.13 1.08mg# 0.05 0.05 0.06 0.02 0.01 0.02Be (ppm) 4.27 4.06 4.38 4.14 15.50 10.10Rb 90 92 85 72 247 278Sr 67 72 82 16 4 5Cs 0.76 0.67 0.58 0.38 0.51 0.83Ba 417 523 506 260 9 10V 1.2 1.5 1.4 < D.L. < D.L. < D.L.Cr < D.L. < D.L. < D.L. < D.L. < D.L. < D.L.Co 0.6 0.8 0.6 < D.L. < D.L. < D.L.Ni < D.L. < D.L. < D.L. < D.L. < D.L. < D.L.Cu < D.L. < D.L. < D.L. < D.L. < D.L. < D.L.Zn 196.9 207.7 196.5 219.7 538.9 522.5Y 56.9 62.8 57.3 66.5 244.0 222.7Zr 647 655 680 709 2445 1926Nb 84.9 86.1 87.4 89.9 414.2 314.3Hf 14.60 14.59 15.09 15.00 61.97 47.95Ta 6.35 6.46 6.30 6.53 28.40 21.11Th 10.5 10.8 10.8 9.5 47.0 43.9U 2.89 2.91 2.93 2.76 11.27 12.42Pb 10.7 18.0 15.9 10.4 57.3 23.9Ga 38.5 39.2 39.4 40.6 48.8 41.8La 69.7 73.2 69.9 74.0 393.5 268.4Ce 148 156 152 167 599 503Pr 16.8 17.5 17.2 18.4 94.3 57.8Nd 67.8 71.8 69.7 74.4 330.4 209.8Sm 14.00 14.90 14.42 15.65 62.74 41.75Eu 2.67 3.02 2.89 2.87 5.64 3.08Gd 12.33 13.60 12.82 14.03 49.10 37.92Tb 1.95 2.11 1.96 2.25 7.50 6.20Dy 11.27 12.24 11.21 13.03 42.25 36.26Ho 2.12 2.28 2.08 2.40 7.95 6.87Er 5.79 6.23 5.58 6.52 22.44 18.81Tm 0.836 0.905 0.799 0.930 3.300 2.671Yb 5.52 5.88 5.27 5.99 21.30 17.10Lu 0.838 0.888 0.797 0.911 3.058 2.457Eu/Eu* 0.65 0.65 0.62 0.59 0.24 0.31Zr/Nb 7.6 7.6 7.8 7.9 5.9 6.1LaN/YbN 8.58 8.48 9.02 8.41 12.57 10.68∑REE 360 381 367 398 1642 1212

FeOt: total Fe as Fe3+; L.O.I.: Loss on ignition; D.I.: Differentiation index; P.I.: Peralkaline index; D.L.: detection limit


137

the volcanic eruptions started at northeastern end of the Line, in the Lake Chad Basin, in Late Cretaceous (~69 Ma). The rhyolites from Mbépit Massif are much younger (45.5 ± 1.1 Ma: Wandji et al. 2008; see Tab. 1); the volcanic manifestations were thus absent till c. 45 Ma, even though minor plutonic complexes were emplaced during this period. Both plutonic and volcanic activity took place between 65 Ma and 30 Ma, recorded between Kapsiki and Bambouto at latitudes of 5–11°N. Volcanic eruptions were clearly predominant along the whole Cameroon Line between 10°N and 1.5°S, from about 25 Ma to recent, whereas minor plutonic intrusions are recorded during this time span at Mts. Rumpi. The most recent eruptions have been witnessed at Bioko and Mount Cameroon, respectively, in 1923 and 2000.

The absence of a progressive age distribution (Fig. 4), which would be expected in the case of a plume (Fitton and Dunlop 1985), lead us to refute the hypothesis of a hot spot for the origin of the “Cameroon Hot Line”, which was previously considered as a Cenozoic to present tectonomagmatic zone (Déruelle et al. 2007; Nkouandou and Temdjim 2011). The situation must be reconsidered on the basis of the new dating of the Lake Chad volcanic rocks.

8.2. Inferences concerning conditions of crystallization

The crystallization of Na-rich clinopyroxene (i.e., aegirine–augite and aegirine) and other Na-bearing phases, such as arfvedsonite, contribute efficaciously to the formation of peralkaline magmas (White et al. 2005). The stability and composition of clinopyroxene and arfvedsonite are temperature-, H2O activity-, melt composition- and fO2-dependent (Scaillet and Macdonald 2001). Anhydrous conditions inhibit aegirine and Na-rich amphibole crystallization; also low fO2 destabilizes Na-rich clinopyroxene that transforms into a fayalite–il-menite (Ronga et al. 2010). The late destabilization of arfvedsonite in the rhyolitic liquids of Lake Chad took place at low temperatures (< 900 °C), as evidenced by the Ca-rich composition of fayalite. Low fO2 values (below QFM buffer) at temperatures less than 800 °C favour the development of peralkaline liquids without the appear-ance of aegirine or sodic amphibole as liquidus phases (Ronga et al. 2010). The destabilization of arfvedsonite phenocrysts (breaking down into fayalite and ilmenite) was probably caused by a sudden increase in fO2 of peralkaline silicic magmas, which promoted the enrich-ment of Na in clinopyroxene and the crystallization of aegirine (Njonfang and Nono 2003). Similar effect was shown also for the aegirine from Djinga Tadorgal and São Tomé phonolites (Mbowou 2009). The occurrence of alkali feldspars with intermediate compositions (anortho-clase–Na-sanidine) provides an evidence for hypersolvus crystallization conditions (Wandji et al. 2008). Nearly pure albite and sanidine are regarded as reflecting alkali exchange during sub-solidus hydrothermal reactions or as corresponding to late-stage recrystallization at hydro-thermal conditions. The resorption rims (corrosion gulfs), seen on quartz phenocrysts in Hadjer el Hamis rhyolitic lavas, indicate a disequilibrium with the melt. They were probably generated during decompression of the rhyolitic magmas, during ascent to the surface upon eruption. Furthermore, given the quartz stability mainly in the low-temperature magma, thermal erosion and fluid-melt infiltration could have also contributed to the occurrence of resorbed quartz.

10

100

LaCe

PrNd

SmEu

GdTb

DyHo

ErTm Lu

Yb

Lava/P

ive M

an

tle

rim

it

Hadjer Hidjer

Hadjer Bigli

Hadjer el Hamis

0

1

10

100

1000

Cs

Rb

Ba

Th

U

Nb

Ta

K

La

Ce

Pr

Pb

P

Nd

Sr

Sm

Hf

Zr

Ti

Eu

Gd

Tb

Dy

Y

Yb

Lu

La

/Prim

itiv

e M

an

tle

av

Fig. 6 Representative primitive mantle-normalized (McDonough and Sun 1995) REE diagrams for peralkaline rhyolites from Lake Chad.

Fig. 7 Representative primitive mantle-normalized (McDonough and Sun 1995) multi-element diagrams for peralkaline rhyolites from Lake Chad.


138

8.3. Implications of various distributions of geochemical data

Silicic lavas from Lake Chad (pantellerites and comen-dites) may be genetically related to each other, cor-responding to a series of progressively differentiated magmas. Progressive depletion in FeOt, MgO, CaO and Co from group-1 to group-2 peralkaline rhyolites indi-cates a simultaneous fractional crystallization of mafic minerals (Fo-rich olivine, and/or clinopyroxene) from a basaltic parental magma. The well-developed negative K, Ba, Ti, Sr and Eu anomalies of the peralkaline rhyolites are consistent with crystallization of arfvedsonite (K, Ti), ilmenite (Ti) and alkali feldspar (K, Ba, Sr, Eu). Further-more, the development of progressively deeper negative Eu anomaly with Eu/Eu* (Eu/Eu*=EuN/√(SmN × GdN)) ranging from 0.24–0.31 in Hadjer Bigli to 0.59–0.65 in Hadjer Hidjer and Hadjer el Hamis, can be explained by major feldspar fractionation. The crystallization of Na–K-rich minerals (e.g., fluoro-arfvedsonite, aegirine–augite, aegirine, alkali feldspars), which fractionate alkalis, con-tributes efficiently to the SiO2-oversaturation of residual silicic magma and likely leads to the crystallization of quartz. Positive Zr anomalies are linked to the inhibi-tion of the zircon crystallization (Watson 1979) or to the increase of the Fe3+/Fetotal ratios during the late stages of magmatic evolution (Duggan 1988; Farges et al. 1994). The HFSE (e.g., Ti, Zr, HREE, Y and Nb) are usually not transported in aqueous fluids, except when these contain high proportion of certain complexing agents such as F–. As shown by Pearce and Norry (1979), F-rich arfved-sonite and F-rich fluids control Zr distribution. Further-more, the observed incompatible elements enrichment (e.g., Rb, Zr, Nb, Th, U and LREE) is consistent with a formation from a highly fractionated residual liquid.

For comparison, peralkaline rhyolites of Kapsiki Pla-teau enclosing small xenoliths of basalt and containing ilmenite, arfvedsonite, aegirine–augite phenocrysts, have high concentrations of Zr (up to 2180 ppm) and Nb (up to 780 ppm) with relatively high Zr/Nb ratios (6.3–8.3) similar to those of Lake Chad rhyolites. Conversely, Benue Valley rhyolites are characterized by a sodic min-eralogy (aegirine–augite, richterite and arfvedsonite) with lower Zr/Nb ratios (~1.7), possibly due to aegirine–augite (up to 1.07 wt. % ZrO2) fractionation (Ngounouno et al. 2003). Mbépit Massif contains phenocrysts of Na–K-feld-spar, quartz and Fe–Ti oxides with higher Zr/Nb ratios (10–11). This indicates that rhyolites similar to those of Lake Chad are scarce within the “Cameroon Hot Line”.

8.4. Genesis of silicic magmas

Contrasting interpretations are proposed to explain the origin of continental-rift peralkaline rhyolites (see Ronga

et al. 2010 and references therein): (1) fractional crystal-lization of a basaltic parental melt slightly contaminated by crust (Peccerillo et al. 2003, 2007) and (2) partial melting of local lower crust triggered by alkali-bearing volatiles influx (Scaillet and Macdonald 2001 and refer-ences therein) and/or basic magma underplating.

Fractional crystallization from basalts to silicic lavas would yield some geochemical characteristics of per-alkaline melts but it fails to explain the lack of rocks with intermediate compositions and the high volume of evolved rocks (Peccerillo et al. 2003). However, the lack of intermediate lavas around the Lake Chad can be re-lated to their high viscosity, preventing them from erupt-ing. Pantellerites and comendites have higher viscosities still but they are associated to much lower density (Ronga et al. 2010).

The peralkaline rhyolites from Lake Chad could not be solely the product of fractional crystallization of ba-saltic melts. Some other processes, such as the transfer of F-rich fluids, could have greatly modified the compo-sition of residual magma. These fluids, whose presence is attested by the occurrence of F-rich minerals (e.g., fluoro-arfvedsonite), could come from a metasomatised mantle source (Dautria et al. 1992; Hauri et al. 1993; McDonough and Rudnick 1998) as suggested for the lithospheric mantle beneath the “Cameroon Hot Line” (Déruelle et al. 2007). However, according to Bohrson and Reid (1997), the peralkaline silicic magmas could be generated in the lower crust, by partial melting of either old crust or young underplated basalts triggered by alkali-bearing volatiles influx. Thus, the occurrence of F-rich minerals could also be linked to such a partial melting of local, fluids-rich lower crust.

9. Conclusions

Petrographic features as well as major- and trace-element whole-rock geochemical data for silicic volcanic rocks from Lake Chad allow us to establish that the silicic volcanic rocks are the products of prolonged fractional crystallization of parental alkali basalts, accompanied by an influx of F-rich fluids suggested by the presence of hydrated F-rich mineral phases (fluoro-arfvedsonite) in these rocks. The fluids came presumably from a metaso-matized mantle source. Peralkaline rhyolites from Lake Chad, with an age of 69.4 ± 0.4 Ma (40Ar/39Ar laser single grain sanidine dating) are likely the first erupted lavas along the “Cameroon Hot Line”.

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an anonymous reviewer are thanked for their positive reviews. V. Kachlík and V. Janoušek are acknowledged for their editorial work, which improved greatly the qual-ity of this manuscript.

Electronic supplementarymaterial. The table of Ar–Ar data on sanidine and overview of previous ages for vol-canic rocks along the “Cameroon Hot Line” is available online at the Journal web site (http://dx.doi.org/10.3190/jgeosci.118)

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